The proper function of the mitochondrion requires the coordinated expression of hundreds of nuclear genes and 37 mitochondrial-encoded genes. Over 100 human diseases have been traced to mutations in mitochondrial DNA or in nuclear-encoded genes important in mitochondrial function. However, variable phenotypes among patients suffering from the same mtDNA mutations have been attributed to other polymorphisms linked on the same mtDNA molecule, or to variable nuclear genetic background. The functional consequences of joint nuclear-mtDNA variation are poorly understood, and no models exist for the systematic analysis of these interactions. The PIs have shown that mtDNA variation has measurable effects on fitness and mitochondrial enzyme activity in Drosophila. These mtDNA effects depend on the nuclear genetic background of the animals assayed, and show evidence for stronger effects in males than females. The goal of this research is to develop a Drosophila model to dissect the quantitative genetics of nuclear-mitochondrial function. A simple but novel extension of quantitative trait locus (QTL) mapping is proposed where phenotypes are mapped in two cytoplasmic environments carrying divergent mtDNAs. Strains of D. melanogaster are characterized that carry mtDNA from D. simulans with over 600 base pair changes from D. melanogaster mtDNA. These mtDNA introgressions result in clear phenotypic effects, providing a strong mitochondrial "allele" for efficient mapping of nuclear gene interactions. There are four specific aims: 1) Introgress D. simulans and D. melanogaster mtDNAs onto different D. melanogaster nuclear backgrounds and determine the effects of these introgressions on fitness, oxygen consumption, locomotor activity and mitochondrial enzyme activity. 2) Map mito-nuclear QTL for these traits using recombinant inbred (RI) lines of D. melanogaster carried on each of two mitochondrial backgrounds: Oregon R and D. simulans. 3) Conduct a quantitative complementation assay of wild chromosomes paired with known OXPHOS mutants in a graded series of mtDNA backgrounds. 4) Determine the interaction of a range of mtDNAs with homozygous viable mutations in nuclear genes encoding mitochondrial proteins. These studies will determine the impact of joint nuclear and mitochondrial variation on the health of individuals in general populations and test the hypothesis that mtDNA defects are more severe in males than females. [unreadable] [unreadable]